Calories in Onions, red

What makes onions so powerful?

Red onions owe their striking color to water-soluble pigments called anthocyanins, specifically cyanidin glucosides. These compounds are the same antioxidants responsible for the hues in blueberries and red cabbage, but in onions, they serve a dual purpose as natural pH indicators. When you slice a red onion and expose it to an alkaline environment—such as hard tap water or baking soda—the anthocyanins shift from red-purple to a bluish-green. Conversely, adding an acid like lemon juice or vinegar turns them a brighter, more vivid magenta. This chemical reaction is why pickled red onions maintain such a brilliant pink color; the vinegar's acetic acid locks in the pigment. Beyond aesthetics, anthocyanins function as powerful antioxidants in the human body, helping to scavenge free radicals. Additionally, like all onions, red onions contain the prebiotic fiber inulin, which feeds beneficial gut bacteria. The sulfur compounds that cause your eyes to water when cutting them, primarily syn-propanethial-S-oxide, are part of the plant's defense mechanism against herbivores, released only when cell walls are ruptured by a knife.